T-Mobile had the best customer service, and most affordable value for their coverage. I expect in no way that AT&T will honor our current service level.

Even though they'll probably keep the contract language, they will change the "meaning". For example, T-Mobile does not force you (too much) into a tethering deal, while AT&T recently sent everyone SMS warnings for that. Or T-Mobile will upgrade your phone for free, even one year early, if you ask nicely. I've never heard AT&T doing such a nice thing.

They'll probably take away the bandwidth from T-Mobile network, and transfer it to server AT&T customers.

Only good GSM provider is now gone, I might probably bite the bullet and switch the CDMA.